Switchmode power converters are known in the art. In order to improve the efficiency of DC power supplies, switchmode power converters are often used. Despite improved efficiency, however, power losses still result in the switchmode power converters as a result of power switching losses, magnetic core losses, copper losses in conductors, and conduction losses in power semiconductors.
An important limitation of conventional power supplies using a switchmode power converter is the requirement that all the DC output power must be converted and handled by the switchmode power converter. Thus, there can be considerable power dissipation and expense associated with these converters, especially at high-power levels.
Whenever off-line rectified AC-to-DC supplies are employed, the line efficiency or power factor is of interest. As will be appreciated, even simple off-line rectified AC-to-DC power supplies will have poor line efficiency due to the low power factor. The combination of a rectifier and filter capacitor allows line current to flow only in short, uncontrolled surges near each peak of the sinewave voltage input. In order to improve power factor, AC line currents must be shaped into a sinewave which closely matches the input voltage throughout the entire input AC sinewave.
Traditional approaches to improving the power factor have typically utilized switchmode power converters which handled all the power delivered to the output. Therefore the size, cost, and complexity of the switchmode power converter was directly related to the output power.
FIG. 1 is a schematic diagram of a standard off-line rectified AC-to-DC power supply 10. The power supply 10 includes an AC line rectifier 12 and an input filter capacitor 14 for providing power to a conventional switchmode power converter 15. The switchmode power converter 15 includes an inductor 16 connected in series with a switch 17. The inductor 16 and switch 17 are connected in parallel across the input filter capacitor 14 and the positive and negative terminals of the rectifier 12. A control circuit 19 modulates the switch 17 between an open and closed position as a function of the voltage across an output filter capacitor 23. As a result, a boosted output voltage is provided through diode 22 across the output filter capacitor 23. The resulting peak current and RMS AC line current represented in FIG. 2, and the Fourier waveforms in FIG. 3, clearly show the poor line efficiency, low power factor, and high harmonic content of the AC line current resulting from the short, high pulses of input current in the circuit of FIG. 1. The odd harmonics of the line frequency are particularly troublesome. It is of interest to note that these current pulses will be produced regardless of the type, design, or control method used in the switchmode power converter because the input filter capacitor 14 determines the magnitude and shape of the input AC line current.
U.S. Pat. No. 5,179,508 to Lange et al. describes a standby boost converter which utilizes a bypass diode. However, power factor control is not possible using the boost converter described in the '508 patent because of an input filter capacitor. Moreover, the boost converter must be sized to supply full, peak load current and voltage.
In view of the aforementioned shortcomings associated with conventional AC-to-DC power supplies using a switchmode power converter, there is a need for a high efficiency power supply which is efficient even at high power levels. Moreover, there is a need for an off-line rectified AC-to-DC power supply having a high power factor without sacrificing overall efficiency. In addition, there is a strong need for such a power supply which provides high power factor, high efficiency operation with minimal circuit size, cost and complexity.